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Stewardship

Stewardship: Reducing the risk of resistance

Stewardship refers to protecting the long term effectiveness of the chemicals and technology used to control pests and weeds in the Australian cotton industry.

Resistance is an outcome of exposing pest or weed populations to a strong selection pressure, such as an insecticide or herbicide. Genes for resistance naturally occur at low frequencies in most populations. Once a selection pressure is applied, such as an insecticide or from a biotechnology trait, resistance genes can increase in frequency as resistant individuals are more likely to survive and produce offspring.

If selection continues, the proportion of resistant individuals may increase in the population until reduced effectiveness of the toxin is observed in the field.

What do you need to know?

Insect Resistance Management Strategy (IRMS)

The cotton industry has implemented an Insecticide Resistance Management Strategy (IRMS) to manage the risk of resistance of all major pests in cotton including aphids, mites and Helicoverpa spp. The IRMS is applicable to both conventional and Bollgard II cotton, is updated annually and is printed in the Cotton Pest Management Guide.

The results from insecticide and miticide resistance monitoring programs carried out each season are used to identify any field scale changes in resistance levels. With industry consultation, these results are used to inform changes to the IRMS.

The IRMS is split into two regions: Northern and Central/Southern, reflecting the different growing seasons from Central QLD through to southern NSW.

The IRMS aims to minimise selection pressure across consecutive generations of key pests, using a combination of tactics:

Since the introduction of the first biotechnology trait in Australian cotton in 1996, biotechnology has become an important feature of production and a key component in breeding programs. By 2014, approximately 95 percent of the cotton planted in Australia contained at least one biotech trait.

Currently there are two broad classes of cotton biotechnology traits which are approved and available in Australian cotton varieties providing either insect protection or herbicide tolerance, or in varieties which are ‘stacked’ with a combination of both traits.

Bollgard II

Bollgard II cotton was introduced to the Australian market in 2005. It contains two genes derived from the common soil bacterium Bacillius thuringiensis (Bt). These genes produce crystalline proteins which give Bt cotton inbuilt protection against the larvae of Helicoverpa spp.

The introduction of insecticidal transgenic varieties into the Australian cotton market has allowed the industry to reduce pesticide use by more than 90 percent. However the potential development of resistance poses a threat to the continued availability and efficacy of Bollgard II cotton.

Screening has revealed that baseline frequencies of resistance in the Helicoverpa population to both the proteins expressed in Bollgard II cotton (Cry1Ac and Cry2Ab) are much higher than originally anticipated. The continued efficacy of Bollgard II cotton is therefore dependent on the effective implementation of the Resistance Management Plan (RMP).

The Bollgard II RMP is based around five key elements that impose limitations and requirements for management. These are the mandatory growing of refuges, control of volunteer and ratoon plants, a defined planting window, restrictions on the use of foliar Bt, and pupae destruction. In theory the combined interaction of all of these elements should effectively slow the evolution of resistance.

Planting windows

The aim of planting windows is to confine crop development and maturity to limit the number of generations of Helicoverpa spp exposed to Bollgard II cotton each season, restricting the selection pressure on key pests to develop resistance.

The start date of the planting window is based on the date that moths are likely to emerge in a region using long-term temperature data. Imposing a start and end to planting date is especially important in warmer regions where pupae do not necessarily enter diapause over the winter and where there is no climatic restriction on when planting can occur.

Mandatory refuges

Refuge crops are planted to generate significant numbers of susceptible moths that have not been exposed to the Bt proteins in Bollgard II.

The current (2014-2015) options for irrigated Bollgard II refuges are 100 percent sprayed conventional cotton, 10 percent unsprayed conventional cotton or 5 percent pigeon pea (relative to the area of Bollgard II cotton grown). In recent years almost 70 percent of refuges have consisted of pigeon pea.

Refuge crops must receive adequate nutrition, irrigation, and weed and pest management (excluding Helicoverpa sprays) so that they remain attractive to Helicoverpa, attracting females to lay eggs in the refuge.

Volunteer and ratoon control

The presence of volunteers within a refuge diminishes the value of a refuge, as some of the moths emerging from that refuge have had exposure to the Bollgard II proteins, potentially leading to an increase in the frequency of resistant individuals in the population.

The good farm hygiene practice of removing all volunteers in and around cropping areas is not only important in removing disease and pest carryover hosts, but also in reducing the resistance risk to Bollgard II technologies.

Pupae destruction

Helicoverpa larvae enter a diapauses phase in the soils as temperatures being to cool and day length decreases in early autumn, allowing the pest to survive in winter months.

Cultivation of the soil between seasons, during the dormancy phase, is an effective way of preventing moths that developed resistance in the previous year from contributing offspring to the population in the following year.

In Central QLD, due to the warmer temperatures and smaller changes in day length, helicoverpa pupae produced late in the season are less likely to go in to diapauses, making pupae busting less effective. Late season trap crops are used as an alternative, and are timed to be at their most attractive after cotton has cut out. Once the cotton has been harvested, the trap crops are destroyed and cultivated to kill the remaining larvae and pupae.

Evaluating the effectiveness of the RMP

CRDC funds a monitoring program of field populations of moths for resistance to Cry1Ac and Cry2Ab proteins. Monsanto Australia operates a separate but complimentary monitoring program.

The results are used to make decisions about the need for modifications to the RMP from one season to the next to manage resistance.

Third generation Bt technology

Monsanto Australia is currently seeking approval from the regulator for third generation Bt technology, based on the same platform as Bollgard II but with a new protein (Vip3A) added. Commercial release is anticipated in 2016-17.

CSIRO has begun performing screens against the new protein in Bollgard 3 and has found that in both Helicoverpa species, the frequency of genes conferring resistance to Vip3a is higher than expected and is much greater than the starting frequencies for Cry2Ab.

Managing resistance to Bollgard II is critical in the lead up to the introduction of Bollgard 3 to ensure the efficacy of this technology is maintained.